1. Field of the Invention
The present invention relates to multilayered containers comprising an intermediate layer of ethylene-vinyl alcohol copolymer (hereinafter referred to as EVOH) and inner and outer layers of moisture resistant thermoplastic resin, and packages having excellent storage capability, thermoformability and container appearance, prepared by filling the container with contents, hermetically sealing it, and then sterilizing the package in hot water or with steam.
2. Description of the Prior Art
Food packages have traditionally consisted of metal cans, glass bottles and various plastic containers, among which plastic containers have become more widely used in recent years for packaging a variety of foods because of their light weight, flexible shapes, high impact resistance and cost competitiveness.
While metal cans and glass bottles have complete impermeability against oxygen, plastic containers permit oxygen to intrude through the wall in an impermissible amount under specific conditions such as retort sterilization, where both heat and moisture act at the same time, the retorting being limelighted as a tool for revolutionarily new distribution systems. Oxygen having intruded into retorted packages deteriorates the contained foods and worsens their flavor and freshness, thus shortening the shelf life of the package.
For the purpose of overcoming those problems, plastic containers utilizing a multilayered structure having an oxygen barrier layer of EVOH laminated with a thermoplastic resin layer via an adhesive layer have become commercially available. These containers still have the following drawbacks and are hence not satisfactory as food packaging containers.
In these multilayered containers, although the EVOH layer shows very good gas barrier properties under relatively low humidity, it rapidly loses the good gas barrier properties when exposed to high temperature and high humidity conditions. Retorting, typically, creates such conditions and causes the container to be attacked by both heat and moisture at the same time.
Japanese Patent Application Laid-open No. 170,748/1982 (U.S. Pat. No. 4,407,897) and Japanese Patent Publication No. 6508/1987 disclose a multilayered structure comprising a protective layer incorporating a drying agent which reduces the influence of moisture upon retorting and thus improves the above drawback. However, since the moisture having intruded into the protective layer upon retort sterilization contacts the gas barrier layer, the once adsorbed moisture can migrate to the gas barrier layer, thereby causing the barrier properties to decrease. Further when the protective layer incorporating a drying agent is, as is often the case, exposed at the edge of the structure, the layer containing the drying agent tends to dissolve out from the edge, whereby the commercial value becomes lowered.
Japanese Patent Application Laid-open No. 242841/1986 discloses a multilayered container comprising a gas barrier layer of EVOH incorporating an inorganic powder such as talc and a layer of a thermoplastic resin provided on at least one surface of the gas barrier layer. With this system, however, a package having excellent storage capability after retorting cannot always be obtained with ease.
EVOH incorporating talc is also known. For example Japanese Patent Publication No. 21,822/1976 (U.S. Pat. No. 3,976,618) and Japanese Patent Application Laid-open No. 143,980/1987 describe a composition of EVOH incorporating talc. However, the references never disclose a technical thought as disclosed in the present invention, that comprises obtaining a container for packaging foods from a multilayered structure comprising a gas barrier layer of the very composition, and, filling it with food and sealing it and treating the package under a condition where heat and moisture act at the same time, to obtain a food package.
Japanese Patent Application Laid-open No. 253,442/1989 (U.S. Pat. No. 4,999,229) describes incorporation of 5-60% of a filler such as talc into a composition comprising EVOH and polyamide. The literature however does not describe the method for satisfying condition (I), which is an important element of the present invention as later described herein, for example further incorporating a small amount of phosphate radical.
Japanese Patent Application Laid-open No. 308,627/1989 (U.S. Pat. No. 4,960,639) and Japanese Patent Application Laid-open No. 178,344/1990 describe incorporating an EVOH of an inorganic powder such as talc or mica in an amount of 5-50% by weight based on the weight of EVOH, preferably 5-30% by weight on the same basis and a phosphoric acid compound such as sodium dihydrogenphosphate or the like in an amount of at least 3% by weight, preferably 5-30% by weight. These references however do not describe about any method for satisfying the condition (I), for example incorporating a small amount of phosphate radical. Furthermore, the above references all do not describe about the necessity that, in the EVOH composition layer of the body wall of a multilayered container, there be multiplicity of regions in which an inorganic filler such as talc incorporated therein constitutes a plurality of substantially 2-dimensional thin layers extending parallel with the container wall surface and laminated with each other and that the inorganic filler in such regions has a weight average flake diameter of not more than 50 .mu. m and a weight average aspect ratio of flake of at least 3.
EVOH is usually used in the form of multilayered structures comprising the EVOH layer, a thermoplastic resin layer and an adhesive layer. When multilayered structures (films, sheets, parisons and the like) produced by various processes are subjected to secondary processing including heat stretching into containers, especially when heat stretched below the melting point of EVOH, the EVOH layer generate many small cracks and local nonuniform thickness areas caused by poor thermoformability, whereby the oxygen barrier properties of the shaped containers are greatly reduced. Further, the containers thus formed cannot be used for packaging foods because of their poor appearance. In particular, multilayered containers prepared by stretching at a temperature below the melting point of EVOH exhibit, after being retorted, a very poor appearance, part of the surface showing orange-skin like pattern, so that these containers cannot be used for packaging foods.
For the purpose of preventing cracks, small voids and the like of the EVOH layer formed when multilayered structures comprising it are heat stretched below the melting point of EVOH, there have been proposed a method which comprises adding various plasticizers to EVOH (Japanese Patent Application Laid-open No. 88067/1978) and a method which comprises blending polyamide resin with EVOH (Japanese Patent Application Laid-open Nos. 141785/1977 and 36412/1983). In the former method, however, in order to sufficiently improve the heat stretchability plasticizers should be added in a large amount of 10 to 20 parts by weight based on 100 parts by weight of EVOH, which decreases the gas barrier properties to a large extent and also decreases adhesiveness between the EVOH layer and other plastic resin layers, so that the method cannot be put into practical use. The latter method cannot be used in practice either, because the obtained containers contain many gels and are discolored presumably due to high reactivity of polyamide with EVOH.
Japanese Patent Application Laid-open No. 5752/1986 describes that an EVOH composition prepared by mixing in the state of solution different ethylene-vinyl acetate copolymers, and then saponifying the mixture yields, by heat stretching, containers having good appearance. The obtained containers however have a disadvantage that their gas barrier properties decrease to a large extent when used under a condition of high temperature and high humidity, such as by retort sterilization as described above.